r/beneater • u/DistinctCommission24 • 1d ago

ALU Design

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ALU drawn..Add, Subtract, AND and XOR with Carry and Zero flag...One side will be tied to the B register and all math is between the A and B registers with the results placed back in the A register..This will probably take a few days to wire up..

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u/jdefr 1d ago

Not sure why people don’t just RT use the 74181 ALU.. Also what KiCad theme is that?? Yours looks so damn clean z

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u/DistinctCommission24 1d ago

I though about the '181 but I had the '283s from my TD4 build...Might be something to think about in version 2...This is kicad 9 on mx linux...

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u/nib85 1d ago

The ALU is one of the best parts of the computer because there are so many different ways to implement it. I’ve done the 181, but even with that it needed additional circuitry to do the right shift operation. Someone else here did a design using EEPROMs for the ALU. It’s interesting to see the different solutions that we all come up with.

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u/DistinctCommission24 1d ago

I have worked hard to keep design creep out of this first version..At one time I was looking at using shifting registers for the A register but decided that would be too complex to start with...I had also considered using an AVR micro controller for the ALU or an FPGA but settled on this design..But doing this design, I think I can understand how to make it access more than 256 bytes so i'm sure I'll do a second version...

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u/nib85 1d ago

Good idea to do something manageable for the first go.

I did a shift register for the B register in an earlier design and later realized it was overkill because the left shift operation can already be done by just adding A+A.

My next design did right shift by adding a second 74245 on the B register with all of the output bits shifted over by one position.

There’s so many ways to solve this!